This application is based upon and claims the benefit of priority from Japanese patent application No. 2008-033008, filed on Feb. 14, 2007, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to a semiconductor device, and in particular, relates to a semiconductor device manufactured using ultrasonic bonding.
2. Description of the Related Art
As semiconductor devices have achieved higher performance, higher operation speed and larger capacity, it has become more important for a semiconductor device to be able to have a plurality of semiconductor chips mounted on a substrate of a limited size. Therefore, the development of a stacked type semiconductor device adopting a laminated structure, in which a plurality of semiconductor chips are stacked on a base substrate in multiple stages, has been in progress.
In a case of manufacturing a stacked type semiconductor device where a plurality of semiconductor chips are stacked, each of the semiconductor chips is electrically connected with the base substrate by wire bonding or the like after being mounted on the base substrate.
There are also other methods of manufacturing a stacked type semiconductor device, such as a method in which a plurality of semiconductor chips are stacked using flexible substrates as interposers. In such a stacked type semiconductor device, each semiconductor chip is electrically connected with the base substrate through the flexible substrate which is a flexible inter-connecting layer that is inserted between each adjacent semiconductor chip.
This flexible inter-connecting layer is capable of reducing possible stress that could be caused by the difference in thermal expansion coefficients between the semiconductor chip and the base substrate, or the like. Thereby, it is possible to prevent cracks from being generated at the connecting portion (cf. Japanese Patent Laid-Open No. 2001-110978).
Japanese Patent Laid-Open No. 2006-278863 discloses a stacked type semiconductor package in which a plurality of packages, each of which including a flexible substrate and a semiconductor chip fixed to the flexible substrate, are stacked. In this case, a wire formed on the flexible substrate is extends from only one side of the semiconductor chip. The end of the extending wire is connected with a wire of the base substrate. Therefore, with this stacked type semiconductor package, high density packaging is made possible while high reliability with a semiconductor device is secured.
In a semiconductor package using flexible substrates as interposers, an electrode part provided at the flexible substrate and an electrode part provided at the base substrate are connected by soldering, ultrasonic bonding, or the like. With ultrasonic bonding, in particular, electrode parts in respective layers can be bonded in a batch around room temperature, and moreover, it is possible to keep the residual stress small after bonding.
FIG. 1 and FIG. 2 are diagrams illustrating how flexible substrate 20, with semiconductor chip 10 mounted thereon, and base substrate 12 are bonded by ultrasonic bonding. FIG. 1 is a sectional view of a semiconductor device, and FIG. 2 is a perspective view of the semiconductor device.
Semiconductor chip 10 is flip-chip bonded to flexible substrate 20 through bumps (not shown). The semiconductor device is being held by stage 31 in FIG. 1.
FIG. 3A is a plane view of a flexible substrate, and FIG. 3B is a sectional view of the flexible substrate. Wires 22 are formed on the both surfaces of flexible substrate 20. Wires 22 on the surfaces of both sides of flexible substrate 20 are electrically connected each other through filled via 24. Gold electrode 23 is formed on a surface of each wire 22. Gold electrodes 23 on the surfaces of wires 22 of flexible substrate 20 are aligned with the positions of gold electrodes 13 on the surfaces of wires 11 of base substrate 12 so that each of gold electrodes 13 and each of gold electrodes 23 contact each other. In this state, ultrasonic wave is oscillated by ultrasonic tool 30, whereby each of a plurality of gold electrodes 13 and each of a plurality of gold electrodes 23 will be bonded to each other in a batch.
FIG. 4 is a plane view of a base substrate where a plurality of wires with different lengths are formed. FIG. 5 is a graphic representation showing positional dependency of bonding strength between gold electrodes 13 of base substrate 12 and gold electrodes 23 of flexible substrate 20 when they are bonded each other by ultrasonic bonding.
FIG. 5 shows bonding strengths measured at four bonding positions 13a, 13b, 13c and 13d shown in FIG. 4. Moreover, FIG. 5 shows a bonding strength at each of these bonding positions as a ratio to a maximum value of bonding strength when the direction of ultrasonic vibration is parallel to the wiring direction.
Referring to FIG. 5, it can be noted that the bonding strength between the gold electrode of flexible substrate 20 and the gold electrode of base substrate 12, when the direction of ultrasonic vibration is parallel to the wiring direction, depends on the length of wire 11 of base substrate 12. As the length of wire 11 of base substrate 12 becomes longer, the bonding strength increases. On the other hand, when the direction of ultrasonic vibration is perpendicular to the wiring direction, dependency of the bonding strength with respect to the length of wire decreases, whereby variation in bonding strength will be reduced. In this case, however, the bonding strength will decrease.
FIG. 6 is a plane view of a base substrate including a plurality of wires with the same lengths extending from gold electrodes. FIG. 7 is a graphic representation showing positional dependency of bonding strength between gold electrodes 13 of base substrate 12, shown in FIG. 6, and gold electrodes 23 of flexible substrate 20, when they are bonded by ultrasonic bonding. In this case, ultrasonic bonding is carried out while the direction of ultrasonic vibration is rendered parallel to the wiring direction.
Referring to FIG. 7, it can be noted that variation in bonding strength is small among bonding positions 13a, 13b, 13c and 13d on base substrate 12. However, in this case, the bonding strength decreases.
As illustrated above, in the case of bonding gold electrodes 13 of base substrate 12 and gold electrodes 23 of flexible substrate 20 by ultrasonic bonding, the length of wires and the direction of ultrasonic vibration have great influence on the bonding strength between gold electrodes 13 and 23.
In ultrasonic bonding, gold electrodes 13 and 23 are bonded by transmitting vibration of the ultrasonic wave to gold electrodes 13 and 23 so as to let gold electrodes 13 and 23 rub one another. Therefore, when there is variation in the stiffness of gold electrodes 13 and in the stiffness of vicinities of them, variation in bonding strength will also be caused among respective bonding portions.
In addition, wire 11 is generally made of metal. Therefore, when wire 11 connected to gold electrode 13 is adhered tightly to base substrate 12, stiffness of gold electrode 13 and stiffness of vicinity of gold electrode 13 will be strong. In other words, when ultrasonic vibration is carried out, wire 11 will be deformed along with base substrate 12, whereby the stiffness of base substrate 12 will be decreased. As the stiffness decreases, ultrasonic energy will be lost and bonding strength at respective bonding portions will decrease, which is a behavior that is recognized as a problem that needs to be considered.
Moreover, a process window with respect to ultrasonic bonding will become smaller, when appropriate bonding conditions are different among individual electrodes 13 due to the influence of the wire length and due to the direction of ultrasonic vibration, etc.